Gabriel and his twin brother Lucian are considered to be among the oldest Carpathians still alive. They are at least 2,000 years old. He is the more "approachable" brother.
When Lucian began to fear that his twin would become vampire, he pretended to be a vampire himself so that Gabriel would hunt him and thus have a reason to live. Gabriel locked them both underground in Paris for two hundred years before the start of Dark Legend.
In that book, Gabriel literally stumbles into Francesca del Ponce. Later he finds out that he had seen her in a village hundreds of years ago without recognizing her as his lifemate. Lonely and hopeless, Francesca had been planning to kill herself. To prevent that, Gabriel purposely impregnates her the first time they have sex.